Propane Tank - Metal Exterior Compartment

If all else fails, you might try to contact one of the TC manufacturers for assistance. My Outfitter has a custom-made metal compartment that (I think) fits your description, with a simple rectangular frame door. I don't know who made it, but I amsure they can tell you if you give them a call.

You can see the door in this picture:
https://lh5.googleusercontent.com/-HbR_ecbhi6I/TmfCvJyEZoI/AAAAAAAAAEc/ob0IKjuTgsw/w800-h600-no/Britney_2310.jpg

The inside is plain metal.